I'm trying to scan some negatives as a contact sheet using PhotoShop CS and a Canon 8400F flat bed scanner. It appears that the only way I can scan a negative and get a positive on the screen is to scan each negative strip using the film carrier, which is not what I want to do. I don't see a filter in PhotoShop that will convert a negative image to a positive. I would appreciate a recommendation for a filter that will do this, preferably free.

papercup - I wasn't clear in my original post. As soon as I click the "negative" option, the scanning software, expecting the negatives to be in a single strip in a negative carrier, gives an error message saying something about the film needing to be in the negative carrier and refuses to go on. It needs an option to allow specifying that the film negatives are in a contact sheet. But it doesn't work that way. I will give the Mavic suggestion a try.

You might try scanning a mask only area of one of the films - that is an area that has the orange mask
but no exposure information. These masked areas can be found on the front or back of the film.
That will give you a base line correction for the mask (no additional color info)

Each film base will be a little different too - so to batch them they will need to be the same
film type ( ie Kodak 400 , Agfa 100 etc ...) or you will still have large differences.